Heterocyclic pyridinium betaines, a new class of second-order nonlinear optical materials: Combined theoretical and experimental investigation of first-order hyperpolarizability through ab initio, INDO/S, and Hyper-Rayleigh scattering

The first hyperpolarizability, beta, of one of the simplest heterocyclic pyridinium betaines, (1-pyridinio)-benzimidazolate, was measured experimentally with the hyper-Rayleigh scattering technique and compared to the calculated values obtained by the semiempirical INDO/S and ab initio molecular orbital methods. Heterocyclic pyridinium betaines are characterized by their zwitterionic character where a negatively charged aromatic electron-donating group and a positively charged aromatic electron-withdrawing group are linked directly. The experimentally obtained \beta\ value, (115 +/- 25) x 10(-30) cm(5) esu(-1), was in good agreement with those derived from the LNDO/S calculations. The first excited state was found to be characterized by a strong intramolecular charge-transfer state, when the electron transition occurs from a negatively charged aromatic electron-donating group (benzimidazole ring) to a directly linked, positively charged aromatic electron-withdrawing group (pyridinium ring). This nature of the excited state would give rise to a large difference between the dipole moments in the excited and ground states and would contribute to enlargement of beta values.